Permanent magnets based on compositions containing iron, neodymium and/or praseodymium, and boron are now known and in commercial usage. Such permanent magnets contain as an essential magnetic phase grains of tetragonal crystals in which the proportions of iron, neodymium and boron (for example) are exemplified by the empirical formula Fe.sub.14 Nd.sub.2 B. These magnet compositions and methods for making them are described by Croat in U.S. Pat. No. 4,802,931 issued Feb. 7, 1989. The grains of the magnetic phase are surrounded by a second phase that is typically rare earth-rich, as an example neodymium-rich, as compared with the essential magnetic phase. It is known that magnets based on such compositions may be prepared by rapidly solidifying (such as by melt spinning) a melt of the composition to produce fine grained, magnetically isotropic platelets of ribbon-like fragments. High coercivity permanent magnets may be formed from these isotropic particles by practices which are known.
Generally, these particles are characterized by a composition which is quite reactive in that when these particles are exposed to the atmosphere, they are oxidized, thereby irreversibly decreasing the magnetic coercive force obtainable from such particles. This shortcoming becomes most apparent at elevated temperatures.
Many attempts have been made to protect these particles from such oxidation, such as by coating the powdered composition with a suitable epoxy resin. Generally, the methods employed for coating these powdered compositions include mixing the composition with an appropriate solvent and then either evaporating the coated solute onto the powdered particles or alternatively precipitating the coated solute onto the powder particles.
However, these prior attempts are characterized by shortcomings since the methods for coating the powdered composition typically do not result in the individual coating of each particle but rather result in the coating of clusters of the powdered composition. During subsequent processing and handling of such powders, the clusters tend to fracture, leaving uncoated surfaces of the powdered composition. This is problematic in that these uncoated surfaces can then oxidize in the atmosphere during subsequent exposure, thereby diminishing the overall integrity of the powdered composition.
Thus, it would be desirable to provide a means for enhancing the oxidation resistance of such permanent magnet particles, which results in the encapsulation of individual particles with a material which is sufficiently oxidation resistant. It would be further desirable if such a means did not detract from the processability of such particles for the subsequent formation of permanent magnet articles from such particles.